Because PSnow is made up of PS4 SOCs and Sony have made no announcements about upgrading it to the PS5 SOCs, they seem focused on using all the silicon in the consoles unlike MS who pledged 12 months ago to begin upgrading their server blades in 2021. Microsoft also talked last year about how they designed Scarlet to specifically work in data centre environments as well as in consoles and have been planning this thing for a very long time. Also the controller incompatibilities between PS4 and PS5 would be problematic. This is one the the reasons why the Series X controller is virtually identical to the Xbox One.
This isn't just something that Sony can easily follow.
Uh, no. Sony has been playing the long game before Microsoft even knew what the game was.
They designed PlayStation 4 (yes, I said FOUR) with the intent that it could also function as a streaming platform.
Remember Remote Play? (Which, by the way, started on PlayStation THREE.)
Remember SharePlay, where you can allow a second player to join your game remotely, even if that player doesn't own the game?
Remember the SHARE button to capture a video stream of past gameplay?
These were all technologies that were included into the PS4, as a result of their acquisition of Gaikai and OnLive technologies and patents.
The controller issue
could be problematic with certain games, but they've dealt with issues like this several years ago when they were streaming PS3 through PS Now to PS Vita and PS TV. They already have this experience under their belt.